Silicon-based photomultipliers are now being developed actively, and development of radiation detectors such as X-ray CT apparatuses using a photomultiplier is also advancing. An example of such an X-ray CT apparatus is a spectral CT apparatus or a photon-counting CT apparatus that detects a spectrum of photon counts in respective units of X-ray energy passed through a subject. X-ray CT apparatuses such as a photon-counting CT apparatus reconstruct a restored image representing linear attenuation coefficients of a subject, based on the degree of attenuation in the spectrum of the X-rays passed through the subject.
A spectrum of X-rays observed by the detector is distorted with respect to a spectrum of the X-rays incident on the detector, due to a fluctuation introduced in a process of converting X-ray energy into observation values, X-ray energy deviation due to noise, or interactions of X-rays with the detector elements, e.g., photoelectric conversion or scattering. The linear attenuation coefficients of a subject calculated based on such a distorted spectrum are therefore not exactly the true values.
Disclosed as an X-ray spectrum detection method for correcting such a distorted spectrum is a technology for acquiring actual X-ray photon count measurements using a subject specialized for calibration, acquiring theoretical values from a simulation, and for acquiring a correction formula for bringing the actual measurements closer to the theoretical values representative of the respective energy units. The correction formula is then applied to a detected X-ray spectrum.
The technology that applies a correction formula, however, is incapable of correcting the photon counts accurately when the subject has a composition different from the condition under which the correction formula is created. In such a case, the pixel values of a sinogram acquired from a spectrum of X-rays passed through the subject may be different from the true values.